Thermal transpiration or thermal creep refers to the thermal force on a gas due to a temperature difference. Thermal transpiration generates a flow of gas in the absence of pressure differences, and maintains a certain pressure difference in a steady state. In most applications, the effect is strongest when the mean free path of the gas molecules is comparable to the dimensions of a container or device.
A well-known device which relies on thermal transpiration is Crookes' Radiometer, also known as a light mill. Generally the light mill is a small chamber containing typically four or more vanes mounted symmetrically around a vertically-oriented axle, with opposing sides of each vane generally parallel to the axle. The parallel sides of the vanes are configured as a high emissivity surface on one side and a lower emissivity surface on an opposite side. When intense light impinges on the vessel, the temperature of the higher emissivity side becomes greater than the lower emissivity side, and the temperature difference generates a force directed toward the colder surface as air molecules contained in the vessel strike on the vanes. See e.g., Scandurra et al., “Gas kinetic forces on thin plates in the presence of thermal gradients,” Physical Review E 75(2) (2007), among others. At low pressure the exerted forces are generally proportional to the temperature gradient on the vane, as well as the mean free path of gas molecules, the density of the gas, cross section of the molecules, and other factors. In light mills where the differing emissivity surfaces occupy opposing sides of the vane, a thermal flow of molecules occurs from the cold to the hot side of the vane, and the reaction to this streaming is a force directed opposite to the temperature gradient, in a direction generally normal to the hotter surface, causing the vanes to revolve around the vertically-oriented axle with the low emissivity surface leading and normal to the plane of rotation. This generates corresponding rotation of the axle about its longitudinal axis.
A variety of devices have exploited the resulting rotation of vertical-surface driven light mills for the generation of electrical and mechanical power. Typically electrical generator configurations have utilized the rotating axle as the prime mover to motivate a generator rotor emitting a magnetic field. See, for example, U.S. Pat. No. 4,410,805 issued to Berley, issued Oct. 18, 1983, and see U.S. Pat. No. 4,397,150 issued to Paller, issued Aug. 9, 1983, and see U.S. patent application Ser. No. 14/288,253, filed by Nutter et al. on May 27, 2014 and published as US 2015/0013337 A1 on Jan. 15, 2015. Other devices have directly utilized axle rotation as mechanical power driving, for example, a torque converter. See e.g. U.S. Pat. No. 4,926,037 issued to Martin-Lopez, issued May 15, 1990. Other devices have exploited the resulting rotation of vertical-surface driven light mills for electrical power generation by providing a magnetic field around the light mill and winding an armature to provide conductors oriented generally parallel and perpendicular to a resulting plane of rotation. See e.g., U.S. Pat. No. 9,106,112 B2 issued to Farquharson et al., issued Aug. 11, 2015. However, all these devices rely on differing emissivity surfaces occupying opposing sides of the vane and result in vertically hung vanes revolving around the vertically-oriented axle with the low emissivity surface leading and normal to the plane of rotation. Due to the reliance on this configuration, the maximum effective length of the rotational force in these devices is on the order of a mean free path length of the surrounding gas, requiring that the opposing high and low emissivity surfaces of these vanes be separated by a thickness substantially equal to this mean free path. As a result, these devices are typically constrained to operate in rarified atmospheres. Further, the vane configurations produce a rotation which constrains the lower emissivity surface to present itself as a significant blunt drag object in the surrounding atmosphere. This tends to impede motion as surrounding pressure is increased and additionally limits the rotational speed which may be achieved, limiting the power generation capabilities of the device.
Thermal transpiration has also been employed to address challenges inherent to miniaturized moving parts, such as micropumps. See e.g. U.S. Pat. No. 5,871,336 issued to Young, issued Feb. 16, 1999, and see U.S. Pat. No. 8,235,675 issued to Gianchandani et al., issued Aug. 7, 2012, and see U.S. Pat. No. 5,611,208 issued to Hemmerich et al., issued Mar. 18, 1997, among others. In these applications, thermal transpiration is employed in a narrow channel to sustain a non-zero longitudinal pressure gradient when subjected to a temperature bias, where the narrow channel has hydraulic diameter smaller than the mean free path of the gas molecules and the temperature gradient is generally parallel to the confining walls of the channel. One accepted physical mechanism which explains the phenomenon posits that an asymmetric momentum transfer between the gas molecules and the channel walls is primarily responsible, since gas molecules from hot areas have a higher average velocity compared to the molecules from the cold side. In Knudsen-type pumps, this asymmetry results in an effective momentum transfer to the channel walls in the direction opposite to the temperature gradient. Although the wall is stationary, this does generate a force parallel to the channel surface, as opposed to generating forces normal to the surface as occurs in the light mills employing vertically mounted vanes.
It would be advantageous to provide a light activated generator which generates thermal creep and corresponding momentum transfers parallel to a vane surface in order to generate rotary motion in response to a radiant flux such as light and directly motivate a conductor through a magnetic flux for the generation of electrical energy. Such a light activated generator would be free of constraints closely tying vane thickness to mean free path lengths, and further would greatly mitigate the impact of light absorbing surfaces acting as blunt force drag objects tending to impede the generated rotary motion. Additionally, such as device would be more amenable to miniaturization allowing potential operation at normal atmospheric pressures.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.